It is known to provide seed drills in which a central seed container is arranged to feed seed to a plurality of row units, each of which comprises a metering device for controlling the dispensed quantity of seed for the attainment of a predefined mutual spacing between seedlings belonging to the respective row.
If the metering device operates with normal air pressure on the material side of the singling disc, an underpressure is generally present on the opposite side thereof for take-up of the seed on the disc prior to dispensing through a purposely designed outlet. The airflow which transported the seed is usually released at the seed containers of the metering devices.
Alternatively, the metering device operates with an overpressure on the material side of the singling disc, which overpressure produces take-up of the seed on the singling disc. This design is especially suitable for precision sowing.
U.S. Pat. No. 5,156,102 shows a device for distributing granular material in a seed drill. The device comprises a drive device which feeds seed from a central seed container to a metering device via a feed duct. An air duct is connected to the feed duct upstream of the connection of the central container to the feed duct.
It is desirable to provide “nursing systems” which allow a good ambient air environment and working environment for the user. Problems which can arise include, for example, that mordant residues are spread by air to vegetation, animals and water courses in the surroundings. It is also desirable to provide precision seed drills which have a reduced power requirement and which are insensitive to disturbances in the compressed air-assisted dispensing of granular material.